The present invention relates to apodizers for redistributing the light intensity of a light beam. More specifically this invention relates to a single component apodizer that can provide a light beam with a flat-top intensity distribution.
Holographic storage systems are storage systems that use holographic storage media to store data. Holographic storage media includes photorefractive materials that can take advantage of the photorefractive effect described by David M. Pepper et al., in xe2x80x9cThe Photorefractive Effect,xe2x80x9d Scientific American, Oct. 1990 pages 62-74.
The index of refraction in photorefractive materials can be changed by light that passes through them. Holographic storage media also include photopolymers, such as those described in Coufal et al., xe2x80x9cPhotopolymers for Digital Holographic Storagexe2x80x9d in Holographic Data Storage, 199-207 (2000), and photochromatic materials. By controllably changing the index of refraction in such materials, high-density, high-capacity, and high-speed storage of information in holographic storage media can be accomplished.
In the typical holographic storage system, two coherent light beams are directed onto a storage medium. The first coherent light beam is a data beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The storage medium records this interference pattern by changing its index of refraction to form an image of the interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a data beam containing the information stored is produced. Most often the appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image.
Information can be encoded within the data beam in a variety of ways. One way of encoding information into a data beam is by using an electronic mask, called a spatial-light modulator (SLM). Typically, a SLM is a two dimensional matrix of pixels. Each pixel in the matrix can be directed to transmit or reflect light, corresponding to a binary 1, or to block light, corresponding to a binary 0. The data beam, once encoded by the SLM, is relayed onto the storage medium, where it intersects with a reference beam to form an interference pattern. The interference pattern records the information encoded in the data beam to the holographic storage medium.
The information recorded in the holographic storage medium is read by illuminating the storage medium with a reference beam. The resulting data beam is then typically imaged onto a sensor, such as a Charge Coupled Device (CCD) array or a CMOS active pixel sensor. The sensor is attached to a decoder, which is capable of decoding the data.
A holographic storage medium includes the material within which a hologram is recorded and from which an image is reconstructed. A holographic storage medium may take a variety of forms. For example, it may comprise a film containing dispersed silver halide particles, photosensitive polymer films (xe2x80x9cphotopolymersxe2x80x9d) or a freestanding crystal such as iron-doped LiNbO3 crystal. U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, generally describes several types of photopolymers suitable for use in holographic storage media. The patent describes an example of creation of a hologram in which a photopolymer is exposed to information carrying light. A monomer polymerizes in regions exposed to the light. Due to the lowering of the monomer concentration caused by the polymerization, monomer from darker unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient creates a refractive index change forming a hologram representing the information carried by the light.
FIG. 1 illustrates the basic components of a holographic system 100. System 100 contains a SLM 112, a holographic storage medium 114, and a sensor 116. SLM 112 encodes beam 120 with an object image. The image is stored by interfering the encoded data beam 120 with a reference beam 122 at a location on or within holographic storage medium 114. The interference creates an interference pattern (or hologram) that is captured within medium 114 as a pattern of, for example, a holographic refractive index grating.
It is possible for more than one holographic image to be stored at a single location, or for a holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 122, depending on the particular reference beam employed. Data beam 120 typically passes through lenses 130 before being intersected with reference beam 122 in the medium 114. It is possible for reference beam 122 to pass through lenses 132 before this intersection. Once data is stored in medium 114, it is possible to retrieve the data by intersecting a reference beam 122 with medium 114 at the same location and at the same angle, wavelength, or phase at which a reference beam 122 was directed during storage of the data. The reconstructed data beam passes through one or more lenses 134 and is detected by sensor 116. Sensor 116, is for example, a charged coupled device or an active pixel sensor. Sensor 116 typically is attached to a unit that decodes the data.
Typically, the data beam and reference beams are provided using a laser illumination system. Beams of light produced by a laser typically have an intensity profile that can be approximated by a Gaussian distribution in which the intensity of the beam varies across the width of the beam (being brightest in the middle and dimmer on the edges).
Accurate data retrieval requires optimal thresholding and detection of the data elements (pixels) by the sensor device. If the reconstructed pixels are not uniform in intensity, the electronics for the sensor will be more complex and less likely to achieve the minimum possible error rate. This will add to the overhead required in the error correction scheme and will ultimately reduce the achievable data capacity of the data storage device.
It is therefore preferred that all of the pixels of the reconstructed data beam have the same intensity. The intensity of the pixels of the reconstructed data beam is dependent upon both the intensity distribution of the light beams used to record the holographic images and upon the intensity distribution of the reference beam used to produce the reconstructed data beam. If the intensity distribution of the data beam encoded by the SLM has a greater intensity in the middle of the data beam, the pixels illuminated by the middle of the data beam will be recorded with a greater intensity than the pixels illuminated by the edges of the data beam. Similarly if the reference beam used to produce the reconstructed data beam has a greater intensity in the middle of the reference beam, the middle of the holographic image will be illuminated with a greater intensity than the pixels stored toward the edge of the hologram. Consequently, using light beams that have a variable intensity distribution to record and reproduce images can produce a reconstructed data beam in which the intensity of the pixels varies within the beam.
Accordingly, a need exists for optical systems that can change the intensity profile of a beam of light to produce a beam of light that has little intensity variance. One approach to producing a light beam with less variance is to over-expand the laser beams and then use only the central part of the beam. The intensity of the center part of a laser beam typically has less variation than the rest of the beam. This approach, however, is inefficient since a large amount of the laser light power is unused. Consequently, more powerful lasers are required to make up for the unused energy.
A more efficient approach to changing the intensity profile of a beam of light is to expand the beam in a nonlinear way, such that light intensity is redistributed within its aperture. Optical systems for redistributing a beam of light in such a way are know as apodizers. Two common types of apodizer systems are known. The first type of apodizer is the Keplerian apodizer. A Keplerian apodizer comprises two positive lens components, with an internal focal plane between the two lens components.
FIG. 2 shows an example of a typical Keplerian apodizer. The Keplerian apodizer shown in FIG. 2 has two lens components 202 and 206. First lens component 202 refracts the light beams 200 towards internal focal plane 204. The refracted light beams 200 then continue toward second lens component 206. Once the light beams 200 have the proper intensity distribution, second lens component 206 recollimates the light beams 200. A Keplerian apodizer is described in detail in John A. Hoffnagle, C. Michael Jefferson, xe2x80x9cDesign and performance of a refractive apodizer that converts a Gaussian to a flat-top beam,xe2x80x9d Appl. Opt., 39, 5488-5499 (2000).
A second type of apodizer system is the Galilean apodizer. The Galilean design comprises a negative diverging lens followed by a positive collimating lens. FIG. 3 shows an example of a typical Galilean apodizer. First lens component 302 refracts the light rays 300 toward the surface of second lens component 304. Once the light rays 300 have the proper intensity distribution, second lens component 304 recollimates the light rays 300. The Galilean apodizer, unlike the Keplerian apodizer, does not have an internal focal plane. A Galilean apodizer is described in detail in J. L. Kreuzer, xe2x80x9cCoherent light apodizer yielding an output beam of desired intensity distribution at a desired equiphase surface,xe2x80x9d U.S. Pat. No. 3,476,463 (Nov. 4, 1969).
The prior art apodizing systems have the drawback of requiring multiple lens components. When multiple lens components are used, the alignment of the various components becomes important, making these designs difficult to fabricate and less robust.
Disclosed are apodizers and methods for redistributing the intensity of a light beam. A preferably apodizer is a single component lens that can convert a light beam with a Gaussian intensity distribution into a light beam with a flat-top intensity distribution.
In one embodiment the method of redistributing the intensity of a collimated beam of light comprises projecting a collimated beam of light through a single component lens having two aspheric surfaces. The single component lens produces a collimated beam of light with a flat-top intensity distribution.
Preferably, the collimated beam of light with a flat-top intensity distribution is projected onto a holographic storage medium or an encoding device. Preferably, a laser produces the collimated beam of light.
Preferably, the single component lens is a single element lens. Preferably, at least 75% of the intensity of the projected light beam is incident upon the single component lens. Preferably, the single component lens has an axial thickness divided by a selected beam diameter of less than 20.
In another embodiment the single component lens for redistributing the intensity of a collimated beam of light comprises a first surface that refracts a collimated light beam entering the single component lens and a second surface that recollimates the refracted light beam. The light beam has a first intensity profile when entering the single component lens and a second intensity profile when exiting the single component lens.
Preferably, the single component has a coupling section that connects the first surface and the second surface. Preferably, the first surface causes light rays within the collimated light beam entering the single component lens to diverge. Preferably, the first surface has a negative radius of curvature.
Preferably, the single component lens has an axial thickness measured from a vertex of the first surface to a vertex of the second surface, the light beam has a selected beam diameter, and the axial thickness divided by the selected beam diameter is less than 20.
Preferably, the first intensity profile is a Gaussian profile and the second intensity profile is a flat-top intensity profile. Preferably, the single component lens has only a single lens element. Preferably, the first surface and the second surface are aspheric.